1. Field of the Invention
This invention relates to conductive polymer compositions comprising a particulate conductive filler which is distributed in an organic polymer.
2. Introduction to the Invention
Conductive compositions comprising a particulate conductive filler distributed in an organic polymer (this term being used herein to include polysiloxanes) are known. Such compositions are known as "conductive polymer compositions". Documents describing conductive polymer compositions and devices comprising them include U.S. Pat. Nos. 2,952,761, 2,978,665, 3,243,753, 3,351,882, 3,571,777, 3,591,526, 3,757,086, 3,793,716, 3,823,217, 3,858,144, 3,861,029, 3,950,604, 4,017,715, 4,072,848, 4,085,286, 4,117,312, 4,177,376, 4,177,446, 4,188,276, 4,237,441, 4,242,573, 4,246,468, 4,250,400, 4,252,692, 4,255,698, 4,271,350, 4,272,471, 4,304,987, 4,309,596, 4,309,597, 4,314,230, 4,314,231, 4,315,237, 4,317,027, 4,318,881, 4,327,351, 4,330,704, 4,334,351, 4,352,083, 4,361,799, 4,388,607, 4,398,084, 4,413,301, 4,425,397, 4,426,339, 4,426,633, 4,427,877, 4,435,639, 4,429,216, 4,442,139, 4,459,473, 4,470,898, 4,481,498, 4,476,450, 4,502,929, 4,514,620, 4,517,449, 4,529,866, 4,534,889, 4,545,926, 4,562,313, 4,570,055, 4,582,983, 4,591,700, 4,624,990, and 4,661,687; J. Applied Polymer Science 19, 813-815 (1975), Klason and Kubat; Polymer Engineering and Science 18, 649-653 (1978), Narkis et al; Japanese Patent Publication Nos. 51-32983, 51-32984, 57-228128, 60-115678 and 61-123665; German OLS No. 2,821,799; European Application No. 38,718; and copending commonly assigned U.S. Ser. Nos. 656,046 (Jacobs et al) now abandoned, published as European Application No. 63,440, 300,709 and 423,589 (Van Konynenburg et al), 832,562 (Masia et al), 735,428 (Jensen et al) now U.S. Pat. No. 4,700,054, 780,524 (Batliwalla et al) now abandoned, 711,910 (Au et al.) now U.S. Pat. No. 4,724,417, 720,117 (Rosenzweig et al.) now U.S. Pat. No. 4,775,501, 720,118, (Soni et al.) published as European Application No. 157,759, 784,288 (Soni et al.) published as European Application No. 220,003, 787,218 (Matthiesen) now U.S. Pat. No. 4,689,475, 913,290 (Barma et al.), 024,738 (Cheng et al.) now abandoned, 021,820 (Siden et al.) now abandoned, 061,353 (McMills), 061,354 (McMills), 064,287 (Wasley et al.) and 061,259 (McMills et al.). The disclosure of each of the patents, publications and applications referred to above is incorporated herein by reference.
Conductive polymer compositions can be used as current-carrying components, eg. in heaters and circuit protection devices, as shielding or stress-grading components for high voltage cables and other high voltage electrical equipment, and as antistatic materials. They may exhibit what is known as PTC (positive temperature coefficient), ZTC (zero temperature coefficient) or NTC (negative temperature coefficient) behavior. The term "PTC behavior" is used in this specification to denote a composition which, in the operating temperature range, eg. 0.degree. to 200.degree. C., has an R.sub.14 value of at least 2.5 or an R.sub.100 value of at least 10, preferably both, and which preferably has an R.sub.30 value of at least 6, where R.sub.14 is the ratio of the resistivities at the end and the beginning of the 14.degree. C. temperature range showing the greatest increase in resistivity, R.sub.100 is the ratio of the resistivities at the end and the beginning of the 100.degree. C. temperature range showing the greatest increase in resistivity, and R.sub.30 is the ratio of the resistivities at the end and the beginning of the 30.degree. C. temperature range showing the greatest increase in resistivity. The term "NTC behavior" is used in this specification to denote a composition which does not show PTC behavior in the operating temperature range, and whose resistivity at 0.degree. C. is at least 2 times, preferably at least 5 times, its resistivity at a higher temperature in the operating range. The term "ZTC behavior" is used in this specification to denote a composition which does not show either PTC behavior or NTC behavior; ZTC compositions can exhibit PTC behavior at temperatures above the operating temperature range of the composition.
The conventional method of preparing conductive polymer compositions comprises dispersing a homogeneous conductive particulate filler in a heated polymeric matrix (the term "homogeneous filler" is used herein to denote a filler in which each particle has a single phase, eg. carbon black, graphite, a metal, a metal oxide, a ceramic or another conductive inorganic material). This conventional method can be used to make a wide variety of products. However, for many combinations of polymeric matrix and conductive filler, it is extremely difficult to obtain reproducible results in some of the resistivity ranges of interest. The reason for this is that the "loading curve", ie. a graph of the log of the resistivity of the composition against the volume per cent of the filler, invariably has a short relatively flat upper portion corresponding to the resistivity of the matrix polymer and then falls steeply until it flattens out as the resistivity of the composition approaches an asymptotic value. Such a loading curve is shown as Curve 1 of FIG. 1. If the desired resistivity falls on the steep portion of the loading curve, the resistivity of the product can change very significantly if there are small changes in the process conditions or the starting materials. For example, a resistivity on the steep part of the loading curve is desired where the conductive polymer is a carbon black loaded, polymeric PTC composition for use in a PTC heater which comprises a laminar PTC heating element sandwiched between laminar electrodes and which is powered by a relatively high voltage (typically greater than 100 V) power source. For such use, a resistivity (at 23.degree. C.) of 10.sup.3 to 10.sup.6 ohm.cm is desirable, inter alia to avoid high and damaging "in-rush" currents when the composition is first powered.
Another known method of preparing a conductive polymer composition is to dry blend carbon black and a powdered polymer, and to sinter the resulting blend so that the polymer particles coalesce but do not lose their identity. Such methods are very useful for the production of ZTC conductive polymers based on polymers which cannot be melt processed, eg. ultra-high molecular weight polyethylene (see for example Ser. No. 720,117, but are not otherwise widely used.
U.S. Pat. No. 3,591,526 (Kawashima) and Japanese Patent Publication Nos. 51-32983 and 51-32984 disclose conductive polymer compositions in which the conductive filler is not a homogeneous material, but rather is a composite filler made by melt-blending carbon black with a thermoplastic polymer to make a PTC composition, and then reducing the blend to finely divided form. The composite fillers disclosed in these references contain high loadings of the carbon black, and the compositions contain high loadings of the composite filler. Consequently the compositions have low resistivities both on an absolute scale (for carbon black containing conductive polymers), ie. of the order of 100-200 ohm.cm or less at 23.degree. C. and as a function of the resistivity of the filler itself, ie. about 10 times the resistivity of the filler or less. The compositions are disclosed as being useful as resistors.